1. Field of the Invention
The field of art to which this invention relates is alumina and its method of manufacture. More specifically, the claimed invention relates to a method of preparing a high surface area alumina particularly suitable for use as a catalyst, by the hydrolysis of aluminum alkoxides. The claimed invention further relates to a method of producing a high surface area alumina in an organic solvent environment by the hydrolysis of aluminum alkoxides.
2. Description of the Prior Art
Alumina has a variety of uses, including catalysts and catalyst supports. Alumina is particularly suitable in applications requiring particles of particular surface area, pore volume and pore diameter. In catalytic applications, the physical and structural properties of the alumina catalyst or support influence significantly the activity and durability of the catalyst. The pore structure of a catalyst, including the pore size distribution and the pore volume, determines to a substantial degree the extent and accessibility of catalytic surface area available for contact of the catalytic material and the reactants. Catalytic activity is often a function of the rate of diffusion of reactants and products in and out of the interstices of a catalyst. Thus, increased pore size may facilitate the diffusion of reactants and reaction products in and out of the interstices of a catalyst and consequently result in increased catalytic activity. However, pore size alone does not influence catalytic activity. Catalytic activity is a function also of surface area available as a reaction site. It is desirable to obtain a catalyst with an optimal balance of surface area and pore size.
The same structural characteristics which influence catalyst activity also can influence physical characteristics of the catalyst, such as durability and density. Generally, catalysts with low density and high crush strength and attrition resistance are desired. Low density catalysts are usually desirable since low density catalysts generally have a low thermal mass and consequently are able to respond more rapidly to temperature changes. High crush strength and attrition resistance is particularly desired of catalysts which are used in large volumes or in moving or fluidized bed operations.
Alumina is a particularly desired catalyst and catalyst support material since it exists in forms with high porosity and surface area, and it is structurally stable over a wide range of temperatures. Alumina has been produced by many processes, including the water of hydrolysis of aluminum alkoxides. However, most processes involving the water of hydrolysis of aluminum alkoxides have produced alumina having surface areas lower than about 280 m.sup.2 /g. Some processes have produced alumina with surface areas as high as about 400 m.sup.2 /g, but the catalyst products of those processes have had a substantially higher macropore volume compared to the method of this invention. Furthermore, the prior art methods which were able to produce aluminas with surface areas of up to about 400 m.sup.2 /g resulted in aluminas with micropore structures very different from the micropore structures of aluminas produced by the method of this invention. (U.S. Pat. No. 3,907,512) For example, to obtain an alumina with a surface area of about 400 m.sup.2 /g, the prior art shows that the resulting alumina has no micropores of a diameter of 40 angstroms or less, and has an average micropore diameter (pores less than 600 angstroms in diameter) of about 150 angstroms. (U.S. Pat. No. 3,987,155)
It has been theorized that high surface area aluminas are difficult to obtain because the surface tension of the gas-liquid interface which forms within the pores of the alumina during drying and calcination causes many pores to collapse. (U.S. Pat. No. 2,249,767)
A method of preparing alumina commonly termed the "aerogel technique" was developed to minimize surface tension forces and thus produces aluminas of up to about 616 m.sup.2 /g. (S. J. Teichner, et. al., Advances in Colloid and Interface Science, 5(1976), 245.) However, the aerogel technique has many serious limitations. Its most fundamental drawbacks are that because it requires the use of very high temperatures and pressures, the technique is complex and the necessary equipment is expensive. The application of the aerogel technique on a commercial scale therefor is very limited.
What has been desired, but not shown by the prior art, is a convenient method of producing an alumina which has a surface area of between about 300 and 700 m.sup.2 /g and an average micropore diameter of between about 20 and 100 angstroms. The method of this invention is such a method.